​What is the third number in a group of three numbers with a combined average of 29, when the average of the other two numbers is 13? ​

Correct option is A

Given:

The combined average of the three numbers is 29.

The average of the first two numbers is 13.

Solution:

Let the three numbers be x, y and z, where x and y are the first two numbers, and z is the third number

The average of the three numbers is given by:

​$$\frac{x + y + z}{3} = 29$$ 

$$x + y + z = 87$$ ......eq (1)

The average of the first two numbers is given by:

​$$\frac{x + y}{2} = 13$$ 

$$x + y = 26$$ .....eq (2)

Substitute the value of x + y in eq. (1)

​$$26+z=87$$ 

$$z=87−26=61$$ 

Thus the third number is 61.